Idle mix screw adjustment
Thread Starter
Instructor
Joined: Jun 2008
Posts: 233
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From: Marietta Ga
Idle mix screw adjustment
Setting up my Quadrajet idle mixture screws I have adjusted one at a time to the highest vacuum reading I can get. I have found that when finished one screw is almost 1 full turn different than the other. Does that matter?
73 350 L82
73 350 L82
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
It's not a 'big' issue. But, that difference may indicate some other condition that causes less vacuum on one side of the engine. It could indicate some valve leakage on one side; or it could indicate a cylinder head-to-intake gasket breach (small one) causing one side to suck excess air in that half of the intake.
In general, it would not be uncommon that the two idle screws were 1/2 turn different. One full turn different gives one pause for considering "Why that much difference?". The carb, and the idle screw tapers and their mating tapered holes, could be most of the cause, as well. (You might want to remove both screws and inspect the smoothness of the tapers on them. If you see damage and/or roughness, you might want to replace or re-dress that screw.)
In general, it would not be uncommon that the two idle screws were 1/2 turn different. One full turn different gives one pause for considering "Why that much difference?". The carb, and the idle screw tapers and their mating tapered holes, could be most of the cause, as well. (You might want to remove both screws and inspect the smoothness of the tapers on them. If you see damage and/or roughness, you might want to replace or re-dress that screw.)
Tech Contributor
Joined: Aug 1999
Posts: 14,380
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From: At my Bar drinking and wrenching in Lafayette Colorado
You can't adjust the idle mixture screws one at a time until best vacuum is achieved - you simply end up with one screw compensating for the other, and they're both actually misadjusted (one will be too rich, and the other will be too lean to compensate). Unless you have a wideband sensor installed in every exhaust port to justify an intentional misbalance between the two, you need to keep both screws adjusted evenly and the same. Start with the same number of turns out from lightly seated (4 turns is a good starting point for a '71), and then adjust them both evenly and the same in 1/2-turn increments until best idle quality is achieved. Best idle quality is usually a touch on the rich side, so if you want to reduce emissions slightly, you can do what GM termed "idle lean drop" adjustment, which means you then turn both screws in (lean) evenly to obtain a 50 rpm drop from best idle. This will give you best CO emissions without running the HC up.
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